I think he was already semi-mastered in the Cell Games, probably didn't fully master it because he felt like he wanted to do his training his own way or some shit like that. Future Trunks was probably too much of a novice fighter to fully master the SSJ form until after he went back to his own timeline.
I also believe SSJ mastery is required to achieve SSJ2 and SSJ3, plus Goten and Trunks are seen to have zero strain from SSJ and have round eyes while in SSJ all the time. I doubt Vegeta's mastery over SSJ was less than that of Goten and Trunks. Vegeta trained in the gravity chamber while maintaining the SSJ form without an aura, I think that was as much of an indication that he mastered SSJ as Gohan managing to suppress himself enough that he wasn't knocking criminals' heads off in Satan City. It also appears that mastering SSJ gives you better training gainz than unmastered SSJ, and I doubt that Vegeta wouldn't capitalize on it